[PATCH 4.9 18/42] slcan: Fix double-free on slcan_open() error path

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Ben Hutchings <ben@decadent.org.uk>

Commit 9ebd796e2400 ("can: slcan: Fix use-after-free Read in
slcan_open") was incorrectly backported to 4.4 and 4.9 stable
branches.

Since they do not have commit cf124db566e6 ("net: Fix inconsistent
teardown and release of private netdev state."), the destructor
function slc_free_netdev() is already responsible for calling
free_netdev() and slcan_open() must not call both of them.

yangerkun previously fixed the same bug in slip.

Fixes: ce624b2089ea ("can: slcan: Fix use-after-free Read in slcan_open") # 4.4
Fixes: f59604a80fa4 ("slcan: not call free_netdev before rtnl_unlock ...") # 4.4
Fixes: 56635a1e6ffb ("can: slcan: Fix use-after-free Read in slcan_open") # 4.9
Fixes: a1c9b23142ac ("slcan: not call free_netdev before rtnl_unlock ...") # 4.9
Cc: yangerkun <yangerkun@huawei.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/net/can/slcan.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/drivers/net/can/slcan.c
+++ b/drivers/net/can/slcan.c
@@ -618,10 +618,9 @@ err_free_chan:
 	sl->tty = NULL;
 	tty->disc_data = NULL;
 	clear_bit(SLF_INUSE, &sl->flags);
-	slc_free_netdev(sl->dev);
 	/* do not call free_netdev before rtnl_unlock */
 	rtnl_unlock();
-	free_netdev(sl->dev);
+	slc_free_netdev(sl->dev);
 	return err;
 
 err_exit: